1. Field of the Invention
The present invention relates to an improved electron gun, and more particularly, to an electron gun having improved thermal characteristics.
2. Description of Related Art
It is well known in the art to utilize a linear electron beam within a traveling wave tube (TWT), klystron, or other microwave device. In a linear beam electron device, an electron beam originating from an electron gun is caused to propagate through a tunnel or a drift tube generally containing an RF interaction structure. At the end of its travel, the electron beam is deposited within a collector or beam dump that effectively captures remaining energy of the spent electron beam. The beam is generally focused by magnetic or electrostatic fields in the interaction structure region of the device in order for it to be effectively transported from the electron gun to the collector without loss to the interaction structure. An RF wave can be made to propagate through a helical structure or set of cavities that comprise the interaction structure, and interact with the electron beam such that the beam gives up energy to the propagating wave. Thus, the electron device may be used as an amplifier for increasing the power of a microwave signal.
The electron gun that provides the electron beam typically comprises a cathode and an anode. The cathode includes an internal heater that raises the temperature of the cathode surface to a level sufficient for thermionic electron emission to occur. When the potential of the anode is sufficiently positive with respect to the cathode, electrons are drawn from the cathode surface and move towards the anode. The geometry of the cathode and anode provide an electrostatic field shape that defines the electron flow pattern. The electronic flow then passes from the electron gun structure through the opening in the anode to the interaction region of the microwave device. Convergent electron guns having spherical cathodes are commonly known as Pierce guns. Electron guns having ring cathodes are commonly known as hollow beam guns.
The typical electron gun is constructed using ceramic structures that provide the functions of mechanically supporting the gun components within a header, electrically isolating the gun components from each other, and providing a wall separating the vacuum environment of the microwave device and the outside world. The ceramic structures often have a cylindrical shape or disk shape. Electrical connections to the gun elements may include metal leads that pass through the ceramic separation structure. These leads may be brazed to the ceramic separation structure in order to form a vacuum seal. Alternatively, the vacuum seal may be provided by metal disks sandwiched with and brazed to the ceramic separation structure. Outside of the vacuum seal, wires may be affixed to the metal leads and the entire region encapsulated by an insulating rubber potting material that prevents high voltage breakdown.
The cathode typically runs at a very high temperature, e.g., around 1,100° Celsius. Some of this heat is conducted through the cathode support structure and the metal leads to the back end of the electron gun header where the leads exit the ceramic separation structure. At this region of the header, the temperature may be approximately 255° C. A drawback with this construction of an electron gun is that the heat can cause the potting material to lose its insulating characteristics (or revert) and thereby allow electrical shorting of the cathode current to the header. This can cause loss of the electron beam and consequent failure of the entire electron beam device. In certain applications requiring high reliability, such as in aerospace or military systems, failure of the electron beam device may render the system inoperative.
Accordingly, it is desirable to provide an electron gun structure having improved thermal isolation to prevent breakdown of the potting material.